This invention relates to photonic devices in general, and more particularly to tunable filters and tunable lasers.
In pending prior U.S. patent application Ser. No. 09/105,399, filed Jun. 26, 1998 by Parviz Tayebati et al. for MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE EMITTING LASER AND FABRY-PEROT FILTER, and in pending prior U.S. patent application Ser. No. 09/543,318, filed Apr. 5, 2000 by Peidong Wang et al. for SINGLE MODE OPERATION OF MICROMECHANICALLY TUNABLE, HALF-SYMMETRIC, VERTICAL CAVITY SURFACE EMITTING LASERS, which patent applications are hereby incorporated herein by reference, there are disclosed tunable Fabry-Perot filters and tunable vertical cavity surface emitting lasers (VCSEL""s).
More particularly, and looking now at FIG. 1, there is shown a tunable Fabry-Perot filter 5 formed in accordance with the aforementioned U.S. patent applications Ser. Nos. 09/105,399 and 09/543,318. Filter 5 generally comprises a substrate 10, a bottom mirror 20 mounted to the top of substrate 10, a bottom electrode 15 mounted to the top of bottom mirror 20, a thin support 25 atop bottom electrode 15, a top electrode 30 fixed to the underside of thin support 25, a reinforcer 35 fixed to the outside perimeter of thin support 25, and a confocal top mirror 40 set atop thin support 25, with an air cavity 45 being formed between bottom mirror 20 and top mirror 40.
As a result of this construction, a Fabry-Perot filter is effectively created between top mirror 40 and bottom mirror 20. Furthermore, by applying-an appropriate voltage across top electrode 30 and bottom electrode 15, the position of top mirror 40 can be changed relative to bottom mirror 20, whereby to change the length of the Fabry-Perot cavity, and hence tune Fabry-Perot filter 5.
Correspondingly, and looking next at FIG. 2, a tunable vertical cavity surface emitting laser (VCSEL) 50 can be constructed by positioning a gain medium (or xe2x80x9cactive regionxe2x80x9d) 55 between bottom mirror 20 and bottom electrode 15. As a result, when gain medium 55 is appropriately stimulated, e.g., by optical pumping, lasing can be established within air cavity 45, between top mirror 40 and bottom mirror 20. Furthermore, by applying an appropriate voltage across top electrode 30 and bottom electrode 15, the position of top mirror 40 can be changed relative to bottom mirror 20, whereby to change the length of the laser""s resonant cavity, and hence tune VCSEL 50.
Tunable Fabry-Perot filters and tunable VCSEL""s of the type disclosed in the aforementioned U.S. patent applications Ser. Nos. 09/105,399 and 09/543,318 are highly advantageous since they can be quickly and easily tuned by simply changing the voltage applied across the top electrode and the bottom electrode.
However,. it has been found that tunable Fabry-Perot filters and tunable VCSEL""s of the type disclosed in U.S. patent applications Ser. Nos. 09/105,399 and 09/543,318 have performance characteristics which can vary slightly from unit to unit. In addition, it has also been found that the performance characteristics of any given unit can vary slightly in accordance with its age, temperature, etc. Accordingly, it is generally not possible to precisely predict in advance the exact voltage which must be applied to a particular device in order to tune that device to a specific frequency. This can present an issue in some applications, particularly telecommunications applications, where the devices may need to be tuned to precise, absolute wavelengths.
As a result, one object of the present invention is to provide a novel wavelength reference apparatus for calibrating a tunable Fabry-Perot filter and/or a tunable VSCEL, whereby the device may be tuned to a precise, absolute wavelength.
Another object of the present invention is to provide a novel wavelength-locking apparatus for tuning a tunable Fabry-Perot filter and/or a tunable VCSEL to a precise, absolute wavelength, and for thereafter keeping that device tuned to that wavelength.
Still another object of the present invention is to provide a novel method for calibrating a tunable Fabry-Perot filter and/or a tunable VSCEL, whereby the device may be tuned to a precise, absolute wavelength.
Yet another object of the present invention is to provide a novel method for wavelength-locking a tunable Fabry-Perot filter and/or a tunable VCSEL, whereby to tune the device to a precise, absolute wavelength, and for thereafter keeping that device tuned to that wavelength.
These and other objects are addressed by the present invention.
In one form of the invention, there is provided a wavelength reference apparatus for use in calibrating a tunable Fabry-Perot filter or a tunable VCSEL to a precise, absolute frequency on a target frequency grid, the wavelength reference apparatus comprising: a first etalon, wherein the first etalon is chosen so as to have its transmission peaks spaced at the target frequency grid; a first detector for detecting the transmission peaks established by the first etalon; a dielectric filter and a second etalon, wherein the dielectric filter is chosen so as to have its transmission peak centered on a peak in the target frequency grid and the second etalon is chosen so as to have its transmission peaks spaced significantly further apart than the target frequency grid; and a second detector for detecting a transmission peak established by the dielectric filter in series with the second etalon; whereby when monotonic light is swept through the apparatus, the transmission peak established by the dielectric filter and the second etalon will identify a specific frequency on the target frequency grid.
In another form of the invention, there is provided a wavelength-locking apparatus for use in tuning a tunable Fabry-Perot filter or a tunable VCSEL to a precise, absolute frequency on a target frequency grid, the wavelength locking apparatus comprising: a first etalon, wherein the first etalon is chosen so as to have its transmission peaks spaced at the target frequency grid; a first detector for detecting the transmission peaks established by the first etalon; a dielectric filter and a second etalon, wherein the dielectric filter is chosen so as to have its transmission peak centered on a peak in the target frequency grid and the second etalon is chosen so as to have its transmission peaks spaced significantly further apart than the target frequency grid; a second detector for detecting a transmission peak established by the dielectric filter in series with the second etalon; whereby when monotonic light is swept through the apparatus, the transmission peak established by the dielectric filter and the second etalon will identify a specific frequency on the target frequency grid; and a controller for tuning the wavelength of the device by monitoring the transmission peaks of the first etalon.
In still another form of the invention, there is provided a method for tuning a tunable Fabry-Perot filter or a tunable VCSEL, comprising the steps of: (1) simultaneously sweeping the wavelength of light output by the device in a monotonic manner through (i) a first etalon so as to generate an inline comb of optical transmission peaks, the first etalon being chosen so as to have its transmission peaks spaced at a desired target frequency grid, and (ii) a dielectric filter and a second etalon, where the dielectric filter is chosen so as to have its transmission peak centered on a peak in the target frequency grid and the second etalon is chosen so as to have its transmission peaks spaced significantly further apart than the target frequency grid; (2) identifying the frequency of the transmission peak of the dielectric filter and the second etalon, and a corresponding one of the transmission peaks of the first etalon; and (3) monitoring the output of the first etalon as the device is tuned so as to tune the device to a desired frequency.